The invention relates to a method of assembling at least one part made of a porous carbon-based material with at least one part made of a copper-rich metallic material, and to an alloy paste for implementing this method.
The assemblies, called refractory assemblies, are assemblies of parts having a use temperature greater than 500° C.
These assemblies are of great interest in the field of thermal engineering. They may be used in the manufacture of heat exchanger components, as they have a high energy exchange density of between 10 MW/m2 and 20 MW/m2. The energy is produced with a very high power on the “carbon-based material side” (which is highly refractory), whereas the heat is recovered on the “copper side” (which is less refractory) by an active cooling system, for example cooling by circulating a coolant.
It is therefore important for this type of assembly to ensure both very good heat exchange and very good mechanical anchoring between the various parts that constitute it.
These assemblies are generally assemblies comprising a part made of a copper-rich material with a part made of a porous carbon-based material.
In the present invention, the term “porous carbon-based material” is understood to mean a material comprising at least 50 wt % carbon, preferably greater than 80 wt % carbon, and most preferably composed of 100% carbon.
This type of material may be graphite, glassy carbon, a composite consisting of carbon fibers in a carbon-based matrix, etc.
Likewise, the expression “copper-rich metallic material” is understood, within the present invention, to mean a material comprising at least 50 wt % copper, preferably a material comprising more than 80 wt % copper, and more preferably pure copper.
Finally, the term “porous material” is understood to mean a material having an open porosity greater than 5% and less than 50% by volume of the material in question.
To produce this type of assembly, methods of assembly have been proposed in which the two parts are bonded together using an organic adhesive. However, the use temperatures of these types of assemblies could never exceed at most 200° C., this being inappropriate for refractory assemblies.
It has also been proposed to produce assemblies by purely mechanical methods, by stapling, screwing, interlocking or riveting. However, these assemblies provide only partial and random contact between the two parts, hence very mediocre heat transfer.
It has also been proposed to assemble the two parts of the refractory assembly by fusion welding, i.e. by applying, at high temperature, pressure at the interfaces between the two parts so as to allow atomic interdiffusion between the two parts. In this method, the temperature must always remain below the melting point of the least refractory material. There is therefore no liquid phase in this system. This type of assembly is carried out either in a press, pressing in a single direction, or in an isostatic chamber. Fusion welding is well suited for assemblies between two metal alloys, but rather unsuitable when there is a refractory ceramic material since the constituent atoms of the ceramic diffuse very little at the joint. However, diffusion welding does not allow partial metal infiltration into the porosity of the ceramic material and therefore it is not possible to obtain both very good heat exchange and very good mechanical anchoring between the two parts.
Thus, to guarantee good heat transfer and good mechanical integrity within the assembly, only methods using a liquid phase, which can partially infiltrate the part made of porous carbon-based compound at the interface, may be envisioned.
For this purpose, several methods use reactive brazing which, in all cases, requires brazing, i.e. melting of the braze shielded from the oxygen in the air, and therefore either in a high vacuum of less than 10−4 mbar or in an inert gas, argon, etc.
Thus, U.S. Pat. No. 5,340,658 proposes to use a brazing alloy consisting of 86 to 99.5% by weight of at least one reactive element, selected from copper, silver, nickel and aluminum, and 0.5 to 10% by weight of an element chosen from vanadium, niobium, titanium, zirconium and silicon as brazing alloy, so as to join a carbon-based compound to a metallic material. In this method, the brazing alloy is placed at the interface between the part made of carbon composite and the part made of metallic material, and the whole assembly is heated in a vacuum at a temperature of between 850° C. and 980° C. for 10 minutes.
It has also been proposed to use brazes called reactive brazes based on silver or copper, which contain such a percentage of reactive elements, such as titanium or zirconium, thereby ensuring good wetting of most ceramic substrates, especially carbon-based ceramics, such as graphite or carbon-carbon composites.
However, the main drawback of reactive brazing is the randomness of the infiltration, which depends in fact on the randomness of the porosity of the carbon-based material. In other words, it is common to observe, at a single interface, zones in which there is strong infiltration of the compound by the braze, which may result in vacancies at the joints, alongside zones which are impregnated too slightly. In this case, the heat transfer will remain mediocre with the possibility of “hot spots” appearing during operation. Equally, the mechanical fastening will be mediocre with the risk of the carbon-based compound debonding from or being torn off its support in the case of a thermal shock.
As a consequence, this fastening, even if it may give useful results, is lacking in reliability for envisaging an industrial application.
To improve this method, U.S. Pat. No. 5,160,090 proposes to machine the surface of the infiltrated porous material so as to artificially increase the area for carbon-based material/braze exchange. This is carried out by laser machining, making regular perforations with conical holes 50 μm to 500 μm in diameter, with a depth of between 100 μm and 2 mm and a space of 0.25 mm between the perforations. This array of perforations makes it possible to ensure that the infiltration of the reactive braze is distributed very homogeneously. The brazes proposed are generally based on silver and copper, these being activated by a few percent of titanium. Although the proposed mechanical surface treatment does solve the problem of reliability of the reactive braze, its use on a large scale still remains problematic because of the cost and implementation time of this operation, despite it being automated.
Moreover, the document “Mechanisms of reactive wetting: the question of triple line configuration”, Acta Mater. Vol. 45, No. 7, p. 3079-3085, 1997, studies the reactive wetting of alloys consisting of 40 at % silicon and 60 at % copper on a glassy carbon substrate or a silicon carbide substrate. This article shows that the wetting with the Cu—Si system on a glassy carbon substrate is good.
However, the wetting alone does not guarantee homogeneous infiltration of the porous substrate, nor good mechanical integrity of a carbon/Cu—Si/copper alloy joint. This is because, for example, excessively good wetting of the braze runs the risk of promoting its complete absorption by the porous carbon-based substrate, which would make subsequent assembly with the copper alloy impossible. Moreover, there is no information about the possible infiltration of the glassy carbon substrate and there is in fact no assembly of a part made of a porous carbon-based material with a part made of a copper-rich metallic material.
The invention aims to alleviate the drawbacks of the methods in the prior art by proposing to use a copper alloy for assembling at least two parts by a method which is industrially applicable while being reliable and inexpensive.